By polymerizing tetrafluoroethylene (hereinafter referred to also as TFE) in the presence of water, a polymerization initiator, a fluorinated anionic surfactant and a paraffin wax stabilizer, it is possible to obtain an aqueous polymerization emulsion of polytetrafluoroethylene (hereinafter referred to also as PTFE) having PTFE microparticles having an average particle size of from about 0.1 to 0.5 μm dispersed (Non-Patent Document 1). Such an aqueous polymerization emulsion of PTFE (hereinafter referred to also as an aqueous PTFE polymerization emulsion) is adjusted in the polymerization so that the concentration of the PTFE microparticles usually becomes a concentration of from 10 to 40 mass % from the viewpoint of the balance between the polymerization yield of PTFE and the polymerization stability of PTFE.
The aqueous PTFE polymerization emulsion is, as it is, likely to be agglomerated (gelled) and thus is unstable. Therefore, it is stabilized by incorporating a nonionic surfactant such as a polyoxyalkylene alkyl ether or a polyoxyethylene alkyl phenyl ether. Further, the fluorinated anionic surfactant is hardly decomposable in the natural world, and a due care is required from the viewpoint of an environmental problem. After stabilizing the aqueous PTFE polymerization emulsion by adding the nonionic surfactant, the aqueous PTFE polymerization emulsion may be passed through a resin column packed with an ion exchange resin to let the fluorinated anionic surfactant be adsorbed on the ion exchange resin thereby to reduce the content of the fluorinated anionic surfactant in the aqueous PTFE polymerization emulsion (Patent Document 1).
And, for the purpose of transportation efficiency or preventing sedimentation of PTFE microparticles, after adding the nonionic surfactant to the aqueous PTFE polymerization emulsion, concentration may be carried out by means of a known concentration method such as a phase separation method or electrophoresis to obtain a highly concentrated aqueous PTFE dispersion wherein the concentration of PTFE microparticles is about from 45 to 70 mass %.
The obtained highly concentrated aqueous PTFE dispersion is used as an aqueous PTFE dispersion, as it is, or after adding water, ammonia or other components, as the case requires (hereinafter the aqueous PTFE dispersion includes the highly concentrated aqueous PTFE dispersion).
One of usages of the aqueous PTFE dispersion is an application wherein to the aqueous PTFE dispersion, a powdery filler such as an inorganic powder, a carbonaceous powder, a metal powder, a heat resistant resin powder, an electrode material for batteries, etc., is added and mixed to coagulate PTFE microparticles, which are then processed to form a lubrication material such as a bearing, a printed board material, an electrode plate material for batteries, etc. (the aqueous PTFE dispersion suitable for such coagulation processing will hereinafter be referred to as the aqueous PTFE dispersion for coagulation processing).
In the coagulation processing, after adding the powdery filler to the aqueous PTFE dispersion for coagulation processing, the aqueous PTFE dispersion is intensely mechanically stirred, whereby PTFE microparticles are coagulated. However, in a case where the concentration of the nonionic surfactant is high in the aqueous PTFE dispersion for coagulation processing, the aqueous dispersion is too stable, whereby there is a problem such that it is difficult to coagulate PTFE microparticles simply by mechanical stirring.
To solve such a problem, a method has been proposed to use a polyvalent cationic coagulation agent such as aluminum ions, iron ions or a polymer coagulant in the coagulation processing of PTFE microparticles, or a method has been proposed to reduce the amount of the nonionic surfactant which is used at the time of stabilizing the aqueous PTFE polymerization emulsion.